Laminated glass has been widely used as a glass that is mounted to windows in transportation systems, e.g., automobiles, railway vehicle and aircrafts, and architectural structures. Such a laminated glass includes a thermoplastic resin sheet, as an intermediate film interposed between a transparent first glass plate or resin plate and a transparent second glass plate or resin plate. Interposition of the thermoplastic resin sheet makes the laminated glass difficult to crack.
The below-specified Patent Literature 1 discloses a laminated glass, particularly for use as an aircraft window glass, which includes two glass plates and a thermoplastic resin sheet composed of PVB (polyvinyl butyral resin) and interposed between them. This thermoplastic resin sheet has a three-layer structure consisting of a PVB internal layer and PVB surface layers disposed opposite sides of the internal layer. Also, the ratio in plasticizer content of the surface layer to the surface layer is adjusted to 1.2-3.
In the laminated glass described in Patent Literature 1, the surface layer is rendered higher in plasticizer content than the internal layer so that the elastic modulus of the surface layer is maintained at a relatively low level. This window glass, because of its use of the above-described thermoplastic resin sheet, is described to exhibit superior impact resistance even at low temperatures. Particularly, the glass plate when chemically strengthened is easier to crack than when thermally tempered, because the former is less compressed than the latter. This has been a problem. However, the use of the thermoplastic resin sheet is described to reduce the occurrence of cracks in the glass plate even when chemically strengthened.
Like Patent Literature 1, the below-specified Patent literature 2 discloses a laminated glass, for use in vehicles, which uses a thermoplastic resin sheet made of PVB resin layers. This thermoplastic resin sheet has a relatively hard PVB inner layer having a Tg of 35-60° C. and PVB outer layers having a Tg of 32-35° C. and disposed on opposite sides of the inner layer. These outer layers are rendered relatively soft as a result of plasticization with addition of a plasticizer.
The Patent Literature 2 describes that the placement of such outer layers on the opposite sides of the above-described inner layer results in the provision of a laminated glass which is stable and exhibits superior mechanical strength.    Patent Literature 1: Japanese Patent Registration No. 3300402    Patent Literature 2: WO 03/097347 A1